FM-CW (frequency modulated continuous wave) radar systems using millimeter radio waves have been developed as in-vehicle radar systems. In the FM-CW radar systems, a radio wave is subjected to frequency modulation within a predetermined frequency range around a predetermined frequency and a beat signal is yielded from a transmission signal and a reception signal. The beat frequency of the beat signal during a frequency-rising modulation period during which the frequency of the transmission signal is rising and the beat frequency thereof during a frequency-falling modulation period during which the frequency of the transmission signal is falling are identified to calculate a distance from the antenna to a target and a relative speed of the target with respect to the antenna.
The frequency modulation is performed at a predetermined angle of inclination during the frequency-rising modulation period and the frequency-falling modulation period to modulate a transmission wave into a triangular modulation wave. Common FM-CW radar systems use voltage controlled oscillators (VCOs) for the modulation of the transmission wave. However, the oscillation characteristics of the VCOs are liable to vary due to the temperature characteristics or due to aging to cause distortion in the modulation waveform. If any distortion occurs in the modulation wave, the angle of inclination of the triangular modulation wave during the frequency-rising modulation period becomes different from that of the triangular modulation wave during the frequency-falling modulation period. In other words, the triangular modulation wave becomes nonlinear.
The triangular modulation wave with any distortion does not provide a sharp peak in the frequency spectrum even if discrete Fourier transform is performed in a manner described below, thus making the detection of the target difficult or causing a significant error.
Conventionally, as shown in FIG. 18, beat signals are sampled, a window function is applied to the sampled data and the discrete Fourier transform is performed to yield a frequency spectrum (analyze the frequency), and any peak included in the frequency spectrum, caused by a reflection signal from the target, is extracted (hereinafter simply referred to “peak extraction”).
FIG. 16(A) shows a triangular modulation wave without distortion. FIG. 16(B) shows the frequency variation of the beat signal during the frequency-rising modulation periods and the frequency-falling modulation periods in the example shown in FIG. 16(A). FIG. 16(C) shows a triangular modulation wave with distortion. FIG. 16(D) shows the frequency variation of the beat signal during the frequency-rising modulation periods and the frequency-falling modulation periods in the example shown in FIG. 16(C).
When the triangular modulation wave is distorted in the above manner, the frequency of the beat signal during the frequency-rising modulation period becomes different from that of the beat signal during the frequency-falling modulation period.
FIG. 17 is a graph showing the respective frequency spectra when the modulation wave is distorted and when the modulation wave is not distorted. Referring to FIG. 17, “a” shows a result when the modulation wave is not distorted as in the example shown in FIG. 16(A), and “b” shows a result when the modulation wave is distorted as in the example shown in FIG. 16(C). Without any modulation distortion, a peak having a very narrow bandwidth occurs because the frequency of the beat signal is not varied during the sampling period. In contrast, with any modulation distortion, a peak having a wider bandwidth occurs because the frequency of the beat signal is continuously varied during the sampling period. As a result, there are problems in that it becomes difficult to detect a target and that a distance cannot be detected with higher accuracy.
Accordingly, methods of applying a control voltage having an inverse waveform (inverse function) with respect to the voltage-frequency characteristics of a VCO to the VCO to make the time-frequency characteristics linear are disclosed in Patent Documents 1 to 3. In addition, a method of correcting the nonlinearity of a VCO in sampling of a beat signal is disclosed in Patent Document 4. Furthermore, a method of applying a frequency control signal corresponding to the temperature characteristics of a VCO to the VCO is disclosed in Patent Document 5.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 7-055924    Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-198833    Patent Document 3: Japanese Unexamined Patent Application Publication No. 8-327728    Patent Document 4: Japanese Unexamined Patent Application Publication No. 7-128439    Patent Document 5: Japanese Unexamined Patent Application Publication No. 10-197625
In the methods disclosed in Patent Documents 1 to 5, a correction bias voltage generated by using a digital-to-analog (DA) converter or a digital signal processor (DSP) is applied to the VCO to generate a highly linear triangular wave. However, since the DA converter or the DSP (an arithmetic processor in the DSP) is required, the system undesirably becomes complicated and increases the cost of the system.
In addition, since it is necessary to calculate compensation values of individual VCOs or to measure the temperature characteristics of individual VCOs to perform the correction, there is a problem in that the measurement, adjustment, and setting works require a large amount of time and effort and increases the manufacturing cost. Furthermore, since a feedback loop is not used in the correction, the correction can involve shifts due to aging. Although the problem due to aging can be resolved if a feedback loop, such as a phase locked loop (PLL), is used in the correction, the circuit configuration becomes complicated and greatly increases the manufacturing cost.